1. Field of the Invention
The present invention relates to a magnetoresistive effect element (MR element) for reading the intensity of a magnetic field, such as a magnetic recording medium, as a signal, a head gimbal assembly (HGA) including the thin film magnetic head, and a magnetic disk apparatus.
2. Description of the Related Art
Recently, in association with high recording density, improvement of performance in the thin film magnetic head is in demand. As the thin film magnetic head, a composite type thin film magnetic head with a structure where a reproducing head having a magnetoresistive (MR) effect element that is exclusively for reading and a recording head having an induction-type magnetic transduction element that is exclusively for writing are laminated is widely used.
At present, as the reproducing head, an MR element with a so-called current-in-plane (CIP) structure that is operated by applying an electric current in parallel with a film surface of the element, referred to as a spin valve GMR element, is widely used. The spin valve GMR element with such structure is positioned between upper and lower shield layers made of a soft magnetic metal film, and, is arranged in a form interposed by insulating layers referred to as gap layers from upper and lower sides. Recording density in a bit direction is determined by a space (reproducing gap space) of the upper and lower shield layers.
In association with increase of the recording density, for the reproducing element of the reproducing head, demand for a narrower shield gap or a narrower truck is stronger. Because of the narrower track of the reproducing element and shortening of element height in association with the narrowing, an area of the element is decreased, but since a heat dissipation efficiency is reduced with the conventional structure in association with the decrease in the area, there is a problem that an operating current is restricted from a viewpoint of reliability.
In order to resolve such problem, the upper and lower shield layers (first shield layer and second shield layer) and the MR element are electrically connected in series, and the GMR element (CPP-GMR element) with a current-magnetoresistive-effect element (CPP) structure not requiring the insulating layer between the shields is proposed, and this is considered as an essential technology to accomplish the recording density exceeding 200 Gbits/in2.
Such CPP-GMR element has a lamination structure including a first ferromagnetic layer and a second ferromagnetic layer formed so as to interpose a conductive nonmagnetic intermediate layer from both sides. The lamination structure of a typical spin valve type CPP-GMR element is a lamination structure where a lower electrode/an antiferromagnetic layer/a lower ferromagnetic layer/a conductive nonmagnetic intermediate layer/an upper ferromagnetic layer/an upper electrode are laminated in respective order from the substrate side.
A magnetization direction of the lower ferromagnetic layer, which is one of the ferromagnetic layers, is secured to be perpendicular to the magnetization direction of the upper ferromagnetic layer when an external application magnetic field is zero. The magnetization direction of the lower ferromagnetic layer is secured by adjoining the antiferromagnetic layer, and by giving unidirectional anisotropy energy (also referred to as “exchange bias” or “coupling magnetic field”) to the lower ferromagnetic layer due to exchange coupling between the antiferromagnetic layer and the lower ferromagnetic layer. Consequently, the lower ferromagnetic layer is also referred to as a magnetization pinned layer. In the meantime, the upper ferromagnetic layer is also referred to as a free layer.
In addition, it is also proposed that a three-layer structure (a so-called “synthetic ferromagnetic (SyF) structure” or “synthetic pinned structure”) with a ferromagnetic layer/a nonmagnetic metal layer/a ferromagnetic layer is adopted to the magnetization pinned layer (lower ferromagnetic layer). This structure enables to provide strong exchange coupling between the two ferromagnetic layers constituting the magnetization pinned layer (lower ferromagnetic layer), and to effectively increase the exchange-coupling force from the antiferromagnetic layer, and further enables to decrease the effect of a static magnetic field to be generated from the magnetization pinned layer on the free layer, as well. Consequently, this “synthetic pinned structure” is widely used at present.
However, in order to respond to the recent demand for the super high recording density, further reduction of thickness of the MR element is required. Under such circumstances, for example, a new GMR element structure having a simple three-layer lamination structure with a ferromagnetic layer/a nonmagnetic intermediate layer/a ferromagnetic layer as disclosed in U.S. Pat. No. 7,019,371B2 and U.S. Pat. No. 7,035,062B1, as a basic structure, is proposed. In this GMR element structure, as shown in FIG. 21, two ferromagnetic layers 61 and 62 are exchange-coupled so as to have their magnetizations 61a and 62a to be antiparallel with each other. Then, a permanent magnet HM is arranged at a depth position, which is opposite from ABS that is equivalent to a recording medium opposition surface of the element, and an initial state where the magnetizations 61a and 62a of the two ferromagnetic layers 61 and 62 are inclined with regard to their track width directions by approximately 45 degrees by a bias magnetic field to be generated from the permanent magnet HM and are substantially at right angles, respectively (see FIG. 22). If the element in this initial magnetization state detects a signal magnetic field from a medium, the magnetization directions of the two ferromagnetic layers 61 and 62 are changed as if the operation of cutting paper with scissors, and as a result, a resistance value of the element is changed. Furthermore, such element structure is referred to as a dual free layer (DFL) element structure in the present specification as a matter of convenience.
When this DFL element structure is applied to the TMR element or the CPP-GMR element, it becomes possible to further narrow “read gap”, which is a space between first and the second shield layers 71 and 72, compared to the general spin valve type CPP-GMR element. Specifically, the antiferromagnetic layer, which is required for the general spin valve type CPP-GMR element, becomes not required, and in addition, the ferromagnetic layer in the “synthetic pinned structure” also becomes not required.
In order to form the DFL element structure in the prior art, it becomes necessary for the two ferromagnetic layers 61 and 62 to be exchange-coupled so as to have their magnetizations 61a and 62a to be antiparallel with each other. Such structure is easily formable by inserting metal, such as Au, Ag, Cu, Ir, Rh, Ru or Cr, between the two ferromagnetic layers 61 and 62, and by generating the exchange coupling between the two ferromagnetic layers 61 and 62.
However, in the TMR element, an insulating film, such as an aluminum oxide (AlOx) film or a magnetic oxide (MgO) film, has to be intervened between the two ferromagnetic layers in order to obtain a tunnel effect, and inconvenience where it becomes difficult to generate strong exchange coupling between the two ferromagnetic layers can occur. As a result, it becomes extremely difficult to bring the magnetizations of the two ferromagnetic layers into the antiparallel state.
Further, in the head structure using the DFL element structure in the prior art, in order to develop bias magnetic field intensity that is sufficient to form the initial state from the permanent magnet HM, such as CoPt, arranged at the depth position that is opposite from ABS, the thickness of the permanent magnet HM has to be thicker. If the thickness of the permanent magnet HM becomes thicker, an advantage where the DFL element structure is a structure that can narrow the read gap cannot be a sufficient benefit. If the thickness of the permanent magnet HM is attempted to be thickened and the read gap is attempted to be narrowed, the space between the permanent magnet HM and first and second shields layers 71 and 72 become smaller, the bias magnetic field to be generated from the permanent magnet HM pass through the first and second shield layers 71 and 72 and application of the bias magnetic field to the element becomes insufficient, and a problem where a resistive change of the element cannot be sufficiently detected can occur.
In addition, in the head structure using the DFL element structure in the prior art, the initial state in the two ferromagnetic layers 61 and 62 is attempted to be formed by arranging the permanent magnet HM at the depth position that is opposite from ABS, and by applying the bias magnetic field from the permanent magnet HM to the two ferromagnetic layers 61 and 62. However, the bias magnetic field from the permanent magnet HM may leak, and problems where a signal is written into the medium due to the leaked magnetic field by mistake, and the signal recorded in the medium may be demagnetized or degaussed also occur.